VARIETIES OF COAL. 153 



cite to graphite. All are but compressed and chemically 

 altered masses of vegetation, the slow fermentation or dis- 

 tillation of which results in the gradual expulsion of the 

 gaseous or volatile portions, and in the retention of the 

 carbonaceous or coaly residue.* The following tabulation 

 exhibits, proximately, this gradation of chemical change by 

 which wood is converted into peat, peat into lignite, lignite 

 into coal, coal into anthracite, and anthracite into graphite : 



(At 212.) Carbon. Hydrogen. Oxygen. Nitrogen. 



Wood 4854 610 35-45 



Peat 5666 5 9 1833 24 1 6 



Lignite 5670 37 1327 10 113 



Coal 70-92 2-6 18 0-2 314 



Anthracite 7494 14 03 trace 17 



Graphite 80-98 ... 17 



Here it will be observed that the gaseous substances, 

 hydrogen and oxygen, so abundant in recent wood and 

 peat, gradually diminish as the mass becomes more and more 

 mineralised, till at length they disappear and leave in con- 

 sequence a gradually-increasing residue of carbon in the 

 true coals, anthracites, and graphites. Like all mixed rocks, 

 however, coal presents itself in many varieties. We can- 

 not conceive of vegetable matter (whether drifted or grown 



* According to M. Fremy, the following are the degrees of alteration 

 of woody tissue : 1. Turf and Peat. Characterised by the presence of 

 ulmic acid, and also by the woody fibres or the cellules of the medullary 

 rays, which may be purified or extracted in notable quantities by means 

 of nitric acid or hydrochlorites, in which they are insoluble. 2. Fossil 

 Wood or Woody Lignite. This, like the preceding, is partially soluble 

 in alkalies, but its alteration is more advanced, for it is nearly wholly 

 dissolved by nitric acid and hydrochlorites. 3. Compact or Perfect Lig- 

 nite. This substance is characterised by its complete solubility in hydro- 

 chlorites and in nitric acid. Alkaline solutions do not in general act on 

 perfect lignites. Reagents in this variety show a passage of the organic 

 matter into coal. 4. Coal. Insoluble in alkaline solutions and hydro- 

 chlorites. 5. Anthracite. An approximation to graphite ; resists the 

 reagents which act on the above-mentioned combustibles, and is only 

 acted on by nitric acid with extreme slowness. 



